test(tui): add TestBackend render tests — connecting, snapshot, no-panic

Closes the 'compiles but never verified to draw' gap:
- render_connecting_state_shows_connecting_text — asserts 'connecting…'
  and 'colibri-harness' title render before daemon connects
- render_with_snapshot_shows_panes_and_agent — asserts pane id, agent
  name, state label, and state icon appear in rendered buffer
- render_does_not_panic_on_empty_snapshot — smoke test for the
  snapshot=None path

All three use ratatui::TestBackend (no terminal needed, CI-friendly).

crates/colibri-glasspane-tui/src/main.rs

@@ -724,4 +724,90 @@ mod tests {
         assert_eq!(app.sessions, vec!["s1", "s2"]);
         assert_eq!(app.session_filter.as_deref(), Some("s1"));
     }
+
+    // ── render tests (TestBackend) ──
+
+    /// Render into a TestBackend buffer and return all visible text as a
+    /// single string — useful for content assertions without pinning exact
+    /// cell positions.
+    fn render_text(app: &mut App, width: u16, height: u16) -> String {
+        use ratatui::backend::TestBackend;
+        let backend = TestBackend::new(width, height);
+        let mut terminal = Terminal::new(backend).expect("terminal creation");
+        terminal
+            .draw(|f| app.render(f))
+            .expect("render should not panic");
+        let buf = terminal.backend().buffer();
+        let mut lines: Vec<String> = Vec::new();
+        for y in 0..buf.area.height {
+            let mut line = String::new();
+            for x in 0..buf.area.width {
+                let cell = buf.cell((x, y)).expect("cell in bounds");
+                line.push_str(cell.symbol());
+            }
+            let trimmed = line.trim_end().to_string();
+            if !trimmed.is_empty() {
+                lines.push(trimmed);
+            }
+        }
+        lines.join("\n")
+    }
+
+    #[test]
+    fn render_connecting_state_shows_connecting_text() {
+        let mut app = App::new(PathBuf::from("/tmp/nonexistent.sock"));
+        let text = render_text(&mut app, 80, 24);
+        assert!(
+            text.contains("connecting…"),
+            "expected 'connecting…' in: {text}"
+        );
+        assert!(
+            text.contains("colibri-harness"),
+            "expected 'colibri-harness' title in: {text}"
+        );
+    }
+
+    #[test]
+    fn render_with_snapshot_shows_panes_and_agent() {
+        let snap = GlasspaneSnapshot::new(
+            "osa",
+            "2026-06-24T12:00:00Z",
+            vec![colibri_glasspane::Pane {
+                id: "pane-1".into(),
+                agent: "zot".into(),
+                state: AgentState::Working,
+                session_id: Some("s1".into()),
+                last_event_at: Some("2026-06-24T12:00:01Z".into()),
+                cwd: Some("/home/clawdie".into()),
+                stalled: false,
+            }],
+        );
+        let mut app = App::new(PathBuf::from("/tmp/nonexistent.sock"));
+        app.snapshot = Some(snap);
+        app.rebuild_session_list();
+        let text = render_text(&mut app, 80, 24);
+
+        assert!(
+            text.contains("colibri-harness"),
+            "expected title in: {text}"
+        );
+        assert!(text.contains("host: osa"), "expected host in: {text}");
+        assert!(text.contains("pane-1"), "expected pane id in: {text}");
+        assert!(text.contains("zot"), "expected agent name in: {text}");
+        assert!(
+            text.contains("Working"),
+            "expected state 'Working' in: {text}"
+        );
+        // State icon for Working is ●
+        assert!(text.contains("●"), "expected working icon in: {text}");
+    }
+
+    #[test]
+    fn render_does_not_panic_on_empty_snapshot() {
+        let mut app = App::new(PathBuf::from("/tmp/nonexistent.sock"));
+        // After refresh, snapshot is still None → "No data" path
+        let text = render_text(&mut app, 80, 24);
+        // Must not have panicked; content is fine either way
+        let _ = text;
+    }
 }